Essay: Spintronics Nirav Parekh_____________________________________________________________________
ABSTRACTThe review paper describes a new era of devices based on spintronics. Spintronics devices exploit the electrons spin or magnetic moment to perform their functions. Unlike conventional charge based semiconductor electronic devices, which works on charge injection, transport, and controlled manipulation, spintronics device specifically exploits spin properties. These properties are exploited by adding the spin degree of freedom to conventional charge based electronic devices or using spin alone to yield potential advantage of non-volatility, increased data processing speed, decreased power consumption, and increased integration densities.INDEX
32. Spin Based Devices
2.1 Giant magnetoresistance (GMR) devices
52.2 Spin transistor
3.Manipulation of Electron Spin
3.1 Generation of spin polarization 8
3.2 Spin injection and spin-polarized transport
3.3 Spin detection
4.Spin Relaxation 125. Conclusion
1. IntroductionSpintronics is a new paradigm in electronics . It is based on exploitation of spin, a quantum property of electron. Therefore, it is called spintonics. Control of electrical properties and modification of information, by spin manipulation, are the two main goals of this field. There are total three categories of spintronics based devices: 1) ferromagnetic metallic alloy based devices, 2) semiconductor based devices and 3) the devices that manipulate the quantum spin states of individual electrons for information processing . Ferromagnetic metallic alloy based devices are mainly used in memory and information storage. They are also termed as magnetoelectronics devices . They rely on the giant magnetoresistance (GMR) or tunnelling magnetoresistance effect. Magnetic interaction is well understood in this category of devices . Semiconductor spintronics devices combine advantages of semiconductor with the concept of magnetoelectronics. This category of devices includes spin diodes, spin filter, and spin FET. To make semiconductor based spintronic devices, researchers need to address several following different problems. A first problem is creation of inhomogeneous spin distribution. It is called spin-polarisation or spin injection. Spin-polarised current is the primary requirement to make semiconductor spintronics based devices. It is also very fragile state. . Therefore, the second problem is achieving transport of spin-polarised electrons maintaining their spin-orientation . Final problem, related to application, is relaxation time. This problem is even more important for the last category devices . Spin comes to equilibrium by the phenomenon called spin relaxation. It is important to create long relaxation time for effective spin manipulation, which will allow additional spin degree of freedom to spintronics devices with the electron charge . Utilizing spin degree of freedom alone or add it to mainstream electronics will significantly improve the performance with higher capabilities .The third category devices are being considered for building quantum computers. Quantum information processing and quantum computation is the most ambitious goal of spintronics research. The spins of electrons and nuclei are the perfect candidates for quantum bits or qubits. Therefore, electron spin and nuclear based hardwares are some of the main candidates being considered for quantum computers . Spintronics based devices offers several advantages over conventional charge based devices. Since magnetised materials maintain their spin even without power, spintronics based devices could be the basis of non-volatile memory device. Energy efficiency is another virtue of these devices as spin can be manipulated by low-power external magnetic field. Miniaturisation is also another advantage because spintronics can be coupled with conventional semiconductor and optoelectronic devices.
However, temperature is still a major bottleneck. Practical application of spintronics needs room-temperature ferromagnet in semiconductors. Making such materials represents a substantial challenge for materials scientists .
2. Spin based DevicesThe present status of spintronics devices at the commercial level is limited to giant magnetoresistance (GMR) based devices. In GMR based memory devises electron spin play passive role . It is limited to detect the change of magnitude of resistance depending on direction of the spin . The change in resistance is controlled by a local or an external magnetic field [2, 12]. But, it is predicted that spintronics can go beyond this passive spin device by integrating electron spin into conventional semiconductors. Thus, the technology based on spintronics may replace conventional semi-conducting devices by introducing active control of electron spin .
2.1 Giant Magnetoresistance (GMR) devices
The read heads in modern hard drives and non-volatile, magnetic random access memory (MRAM) are the two application of GMR effect.In 1988, Albert Ferts group discovered GMR effect. They observed that when multi layers of alternate magnetic/non-magnetic materials carrying electric current were placed in magnetic field, they exhibit large change in electric resistance, which also known as magnetoresistance .
Figure 1 Giant magneto resistance effect; (a) electron transport takes place when magnetization direction of both ferromagnetic regions aligned parallel to each other, (b) electrons are facing high resistance and scattered away near interface when magnetization direction of both ferromagnetic regions are opposite to each other (b).The change in resistance depends on the relative orientation of the magnetization in magnetic layers . The resistance to passage of current is low when the ferromagnetic layers align in the same direction and transfer of current takes place dynamically (fig 1 (a)). If they align themselves in opposite directions electrons scattering occurs near interface and a high resistance path is produced  (fig 1 (b)). The relative orientation of magnetic layers can be altered by the applying external magnetic field . This effect is called spin-valve effect . These multi layers are used to configure the GMR devices. The read heads in hard disk drives utilize spin-valve effect to read data bits. The data bits are stored as the minute magnetic areas on the surface of HDD . Zero is stored, when the magnetic layers align themselves in one direction and one when they align in opposite directions . The read head reads the data by sensing a change in voltage corresponding to a change in resistance . It reads 1 when resistance is higher and 0 when resistance is lower . Thus, the ability of read head to sense minute changes in voltage corresponding to small changes in magnetic fields will allow data storage at highest packing densities in small magnetic particles . The expected value of storage densities may reach to 100 gigbites per square inch by using synthetic Ferromagnets . There are three types of GMR. 2. Spin transistors
The spin-transistors exploit electron spin either by spin-valve effect or by active control of electron spin . The design of transistor is similar to that of GMR devices. It consists of three layers, out of which the non-magnetic layer is sandwiched between the two ferromagnetic layers . Johnson was the first to propose about spin-valve transistor. As per him, the first magnetic layer acts as a spin injector or emitter while the second acts as a spin detector or collector . The non-magnetic layer acts as a base . The magnetization direction of the collector can be changed by the application of an external magnetic field . When the voltage is applied across the emitter-base, it generate electrons with either spin-up or spin-down . When the magnetization direction of emitter and collector is parallel, the current can flow throw the base to the collector . The electrons face high resistance when the relative magnetization direction is opposite. Thus, device acts as one-way switch . Electron spin plays passive role in Johnsons spin-valve transistor.
Figure 3 Dutta-Das field effect transistor; at zero gate voltage, electron preserves spin state in transport channel (a) it enables current flow from source to drain. With applied gate voltage, electrons change their spin state from parallel to anti parallel to the direction of magnetization of ferromagnetic layer (b) this offers high resistance to flow of current. Therefore, electron scattering occurs at drain and no current flow from source to drain . The first model of transistor using active control of electron spin was proposed by Datta and Das. In the Datta-Das field effect transistor, the non-magnetic layer acts as a gate while two ferromagnetic layers act as source and drain respectively (fig 2(a)) . The gate plays an important role in Datta-Das field effect transistor. The gate modifies electron spin by generating effective magnetic field and thereby in switching the transistor . When voltage is applied to the gate, it generates effective magnetic field (fig 2(b)). Thus, by modifying gate voltage one can modify electron spin . The electrons ballistically transport in transport channel, if its spin is parallel to the magnetization direction of dr